Variable resistor



L. GINSBERG Dec. 3, 1968 VARIABLE RES I STOR Filed March 24, 1967 INVENTOR. LEON GINSBERG 3Y-l ATTORNEYS United States Patent 3,414,862 VARIABLE RESISTOR Leon Ginsberg, Weston, Ontario, Canada, assignor to Precision Electronic Components Ltd., Toronto, Ontario, Canada, a company Filed Mar. 24, 1967, Ser. No. 626,375 Claims. (Cl. 338180) ABSTRACT OF THE DISCLOSURE A miniature, hermetically sealed, variable resistor or potentiometer is provided with a lead screw drive to a contact carrier, the contact carrier being hollow and provided with an internally positioned resilient member engaging the lead screw. The resilient member engages the lead screw on opposite sides and is provided with spaced members (47) which, when the carrier is in the limit of its travel, can ratchet over the threads of the lead screw if it is further rotated in a direction tending to move the carrier beyond its limit of travel.

This invention relates to variable resistors and potentiometers and, in particular, to a drive arrangement by means of which a lead screw may be conveniently employed to adjust the position of a movable contact in association with a resistance element.

BACKGROUND OF THE INVENTION Lead screw operated variable resistors and potentiometers are not broadly new. Indeed, devices of this kind in various sizes and of varying constructions have been well known in the art for a considerable length of time. Miniature lead screw operated variable resistors and potentiometers, with which this invention is particularly concerned, embody, for the most part, certain common features of construction and present certain common problems and difficulties. Among these difficulties is the requirement that the complete variable resistor or potentiometer be hermetically sealed and enclosed with the result that it is impossible, from the outside of the device, to determine the position of the movable contact which is adjusted by rotation of the lead screw. Accordingly means must be provided to ensure that continued rotation of the lead screw in one direction will not cause damage to the internal components of the device when the contact moved by this rotation has reached one limit or the other of its travel axially of the lead screw. Numerous constructions have been proposed to overcome this difiiculty with varying degrees of success.

SUMMARY OF THE INVENTION The present invention proposes to overcome the difficulty referred to by providing a novel and particularly eifective arrangement by means of which a movable contact can be caused to axially traverse the length of a lead screw within a variable resistor or potentiometer housing and which will ensure that continued rotation of the lead screw in one direction will not cause damage to the movable components after these components have reached their limit of travel in one direction or the other.

Further, the arrangement which is provided according to the present invention provides for a drive system connecting the lead screw and the movable components which is virtually backlash-free and which is believed to be less subject to deterioration as a result of prolonged use.

Further advantages of the present invention will become apparent as the following description proceeds with reference to the accompanying drawings in which like reference numerals denote like parts in the various views in which:

Brief description of the drawings FIGURE 1 is a perspective view, partly cutaway and exploded, of a variable resistor or potentiometer constructed according to the present invention;

FIGURE 2 is a longitudinal section view of the assembled device of FIGURE 1;

FIGURE 3 is a partial longitudinal section view of the device showing the movable contact carrier in one extreme position;

FIGURE 4 is an enlarged view of a portion of FIGURE 3 illustrating the action of the device under continued rotation of the lead screw after the movable elements have reached the limit of travel, and

FIGURE 5 is an overall perspective view of the completed and assembled device.

Referring now to the drawings and, in particular, to FIGURE 1, the variable resistor or potentiometer of the present invention will be seen to comprise a housing which is generally indicated by reference character 10. The housing is conveniently a plastic moulding and embodies an internal cavity 11 which is defined by a bottom wall 12, two side walls 13, two end walls 14 and which is open along the remaining side of the housing 10. One end wall 14 is provided with an aperture 15 which is counterbored at 16 to provide a shoulder or seat 17. The opposite end wall 14 is internally provided with an upwardly opening recess 18.

A lead screw 19 is provided having an enlarged and slotted head 20 which is carried by one end of a shank 21 of slightly smaller diameter than the head 20. A groove 22 is formed in the shank adjacent the head 20 and the major portion of the remainder of the length of the lead screw is provided with screw threads as indicated at 23. An unthreaded portion 24 extends between the groove 22 and the beginning of the threads 23- and another unthreaded portion 25 lies adjacent the free end 26 of the lead screw 19.

The lead screw 19 is adapted to be inserted within the housing 10 through the aperture 15 so as to lie within the cavity 11 and to be rotatable therein. One end of the lead screw will be supported by the bore 15 and the other end will be supported by the bight of the U-shaped recess 18 in the opposite end wall 14. A resilient sealing member such as O-ring 27 is adapted to lie within the counterbore 16 and to bear upon the seat 17 and a metallic washer 28 bears against that side of the O-ring 27 remote from the seat 17 and also bears against the enlarged head 20 of the lead screw 19 when it is in the position shown in FIGURE 2. A retaining clip 29 is provided with a resilient aperture 30 which, when the lead screw 19 has been inserted within the housing 10, can be moved so that the resilient aperture 30 will enter the groove 22 and embrace the lead screw so as to retain it in position. The position of the groove 22 along the length of the lead screw is such that when the element 29 is engaged therewith, the O-ring 27 will be under compression so as to provide for a hermetic seal about the lead screw and around the bore 15.

A hole 31 is provided in the element 29 by means of which it may be engaged by a hooked tool to facilitate its removal.

The upper edges of the side walls 13 and end walls 14 which define the cavity 11 are provided with an inwardly extending shoulder 32 which constitutes a seat for a planar cover 33 which is adapted to fit within the open side of the cavity 11 and rest upon the shoulder 32. The cover 33 may be of ceramic or plastic material and will carry, upon that face which is presented to the interior of the cavity when the cover is in position, a resistance element 34 and a collector track 35. Terminal elements 36 are provided, in a well known manner to provide electrical connection between the resistance element 34, the collector track 35,

and the outer face of the cover 33. This arrangement can be clearly seen in FIGURE 2.

The thickness of the cover 33 is less than the distance between the shoulder 32 and the upper edge of the side and end walls 13 and 14 respectively. Thus, when the cover is seated upon the shoulder 32, a recess remains above it and into this recess is forced a resilient locking piece illustrated generally at 37 in FIGURE 1. The locking piece 37 is a resilient metallic member of generally rectangular configuration and provided, on one of its longitudinal edges, with a single sharp projection 38 and, on its opposite longitudinal edge, with a pair of spaced sharp projections 39. The projections 38 and 39 are so dimensioned that the locking piece is an interference fit within the open side of the cavity 11. Thus, the locking piece 37 may be forced into the recess above the cover 33 so that the sharp projections 38 and 39 will bite into the plastic material of which the housing has been moulded and provide for a relatively firm mechanical fixing of the cover in position. Once the cover has been thus positioned and locked with the locking piece 37, an adhesive sealant or potting compound may be applied (see FIGURE to seal the cover in position, to hermetically enclose the cavity 11 and to provide additional mechanical support for the terminal wires 36 which project from the cover.

Within the cavity 11 and at least partly embracing the lead screw 19 is a movable contact carrier 46 supporting a contact member 41 which, in the assembled condition of the device (see FIGURE 2) is in a position to electrically engage both the resistance element 34 and the collector track 35 to provide for the adjustable electrical connection between the resistance element 34 and the collector track 35 in a manner which is well understood and which is common to a number of devices of this general type.

The contact carrier 40 is a small, rectangular, plastic moulding adapted to fit snugly within the cavity between the parallel side walls 13. The fit of the carrier 40 within the cavity, however, is sufiiciently loose to enable it to move freely in a direction axially of the lead screw. The carrier 40 is adapted to slide along wall 12 of the housing and is urged into abutment with wall 12 by the resiliency of the contact fingers 41. Thus, as seen in FIGURE 2, the constant engagement between contacts 41 and the cover at the top and the carrier 40 and wall 12 at the bottom hold the carrier firmly, prevent any tendency for it to rotate with the lead screw 19 and provide for a smooth functioning of the movable components.

In order that the carrier 40 may at least in part embrace the lead screw, it is provided with a pair of spaced, parallel walls 42 each of which is provided with an aperture 43 to enable the lead screw to pass through both walls 42 of the carrier 40 as can be clearly seen from FIGURES 2 to 4 inclusive.

Within the contact carrier 40 is a resilient member lying between the spaced, parallel walls 42, the resilient member being in engagement with the lead screw 19 and constituting the drive mechanism by means of which the contact carrier and, hence the contact 41 can be moved axially of the lead screw within the cavity 11 of the housing 10.

The nature and construction of this resilient member can best be seen from FIGURE 1 in which it is shown to comprise a central portion 44 which is integrally joined by means of curved resilient portions 45 to a pair of spaced, parallel and opposed end members 46 which are adapted to lie against the adjacent faces of the spaced, parallel walls 42 of the contact carrier 40. The end members of the resilient element each have, integral therewith, a transverse bar 47 and the arrangement shown in FIGURE 1 is such as to enable the resilient member to embrace the lead screw (see FIGURE 2) with the central member 44 bearing against the lead screw on one side and the transverse bars 47 engaging the lead screw on the diametrically opposite side.

The function of the central portion 44 hearing against the lead screw on one side is to maintain the transfer bars 47 in engagement with the screw threaded portion of the lead screw on its diametrically opposite side as can be seen in FIGURE 2. This engagement of the transverse bars 47 with the screw threads of the lead screw enables them to transform the rotary motion of the lead screw into axial motion in the resilient member which, in turn, being snugly received within the carrier 40, causes the axial translation of the carrier 40 relative to the lead screw upon rotation thereof within the cavity 11.

The function of the apparatus as shown in FIGURE 2 is believed to be obvious and apparent from the drawings during that portion of the movement of carrier 40 which takes place with both of the transverse bars in engagement with the screw threads upon the lead screw 19. Continued rotation of the lead screw in one direction, however, will cause the movement of the carrier axially of the lead screw until one wall 42 abuts one end wall 14 defining the cavity 11 in the housing 10. In FIG- URE 3, the device is illustrated in such a position. The carrier 40 has moved to the right (in FIGURE 2) until one wall 42 of the carrier 40 is in engagement with the internal surface of end wall 14. It is to be observed that the unthreaded portion of the lead screw 19 is now engaged by one of the bars 47 and rotation of the lead screw 19 is, accordingly, as far as this bar 47 is concerned, ineffective to move the carrier axially of the lead screw in either direction. The other transverse bar, however, identified in FIGURE 3 as 470 is still in engagement with the threaded portion of the lead screw 19 and continued rotation thereof will produce the effect shown sequentially in FIGURES 3 and 4.

Turning now to FIGURE 4, rotation of the lead screw 19 in the direction of the arr w 50 after engagement of the carrier 40 with the end wall 14 has caused the curved portion connecting bar 47a with the central portion 44 to flex and permit transverse bar 47a to be moved in a direction parallel to a diameter of the lead screw 19 and away from the central portion 44 so that it may climb up the incline of the screw thread 51 as shown in FIGURE 4.

The effect of further continued rotation of the lead screw 19 in the direction of the arrow will be to allow bar 47a to ride down the side of thread 51 into the trough again. This movement of bar 4711 is made easier by the component of force acting upon it in a direction axially of the lead screw which tends to reduce the frictional engagement between bar 47a and the adjacent end wall 42 of carrier 40. The sequence of events which has been illustrated in FIGURES 3 and 4 will repetitively continue so long as the lead screw 19 is rotated in the direction of the arrow 51} and this continued rotation will result in nothing more than the resilient travel of the transverse bar 47a from trough to crest of the thread 51 (as shown in FIGURE 4) back to the position shown in FIGURE 3. It is apparent that these sequence of events will cause no damage to the internal mechanism of the device.

In the event that the direction of rotation of the lead screw 19 is reversed, the tendency of the transverse bar 47a to climb up upon the thread 51 will be resisted by the frictional engagement of the bar 47a and the end members 46 with the internal face of the adjacent wall 42 of the carrier 40. This frictional engagement is assisted by the resilient action of the curved portions 45 which tend to urge the bars 47 and the end members 46 into abutment with the adjacent faces of the walls 42 within the carrier 40. This resilient engagement precludes and eliminates any backlash which might otherwise exist between the lead screw 19 and the mechanism constituting the movable carrier 40 and the resilient member carried within it. Thus, reverse rotation of the lead screw 19 will immediately produce movement to the left (as seen in FIGURE 2) of the carrier 40 by virtue of the engagement between the transverse bar 47a and the threads on the lead screw 19. The other transverse bar 47 (see FIGURE 3) which is in engagement with the unthreaded portion 25 of the lead screw 19 is, during this initial movement, ineffective to either assist in or detract from the movement of the carrier 40. A few turns of the lead screw 19, however, will bring the right-hand bar 47 into engagement with the threaded portion of the lead screw and, thereafter, both bars 47 will be in engagement with the threaded portion although the movement of the carrier 40 will rely solely upon the engagement between bar 47a and the threads since this is the bar which the rotation of the lead screw will urge to engagement with the wall 42 and which will, therefore, transmit the axially directed force to the carrier 40.

It will, of course, be apparent that continued rotation of the lead screw in a direction such as to move the carrier to the left in FIGURE 2 will utimately bring the carrier into contact with the opposite end wall 14 adjacent which the lead screw 19 is provided with an unthreaded portion 24 equivalent to the unthreaded portion 25 at the opposite end. Here, the sequence of events illustrated in FIGURES 3 and 4 will be repeated in exactly the same manner as before except that, in this event, it will be the opposite bar 47 which will climb up upon the thread 51 in a manner similar but opposite to the manner which has already been described.

It will be apparent that in order for the device illustrated to be operative, the axial extent of the unthreaded portions 24 and 25 must be less than the separation between the internal surfaces of the walls 42 of the carrier 40 so that, at all times, one bar 47 will remain in engagement with the threaded portion of the lead screw.

The advantages to be derived from the construction disclosed herein will be apparent to those skilled in the art, The freedom from backlash which the resilient member provides is an important advantage and, further, another important advantage derives from the fact that the resilient member may be considered as floating within the carrier 40 and that it does not transmit to the carrier 40 any of the flexing which take place when the carrier 40 is at a limit of its travel and the lead screw is further rotated.

While a preferred embodiment of the invention has been described in detail with reference to the accompanying drawings it is to be appreciated that minor modifications in construction and arrangement are contemplated within the spirit of the invention and the scope of the appended claims.

What I claim as my invention is:

1. In a variable resistor comprising a housing, the housing having an internal cavity open along one side of the housing, a threaded lead screw passing through the housing and rotatable in the cavity, the lead screw being screw threaded over a major portion of its length within the cavity and unthreaded adjacent each end of the cavity, a cover adapted to close the housing, a resistance element parallel to the lead screw and presented to the inside of the cavity, a contact carrier at least partly embracing the lead screw and carrying a contact to engage the resistance element, the contact carrier comprising a rectangular element having a rectangular cavity therein defined by at least two spaced, parallel end walls normal to the lead screw, through which the lead screw passes, a resilient element within the rectangular cavity and having a first central portion bearing on the lead screw on one side thereof and having a pair of spaced, parallel opposed end members, integrally joined thereto by means of curved, resilient portions, the end members engaging the lead screw on the side thereof remote fromv the first central portion, the end members also resiliently bearing against the said two spaced, parallel end walls of the cavity in the contact carrier to cause rotation of the lead screw to be transformed into axial movement of at least one of the spaced, parallel end members which, being in direct contact with the adjacent end wall of the contact carrier thus causes movement of the contact carrier axially of the lead screw.

2. In a variable resistor, the arrangement as claimed in claim 1, wherein the length of each unthreaded portion of the lead screw within the cavity is less than the separation between the spaced, parallel end walls of the contact carrier.

3. In a variable resistor, the arrangement of claim 2, wherein the central portion of the resilient member is joined to the end members by curved portions integral both with the end members and the central portion, the curved portions providing resiliency to enable the bars transverse to the lead screw to move away from the central portion by a distance at least equal to the height of a screw thread on the lead screw.

4. In a variable resistor, the arrangement of claim 1, wherein the cavity has a shoulder surrounding the open side upon which the cover may be seated.

5. In a variable resistor, the arrangement of claim 4, wherein the shoulder is spaced from the edge of the open side by a distance greater than the thickness of the cover so that, when the cover is inserted, a recess is defined to receive a locking piece, the locking piece comprising a fiat member of resilient material having two spaced pointed projections on one side and a single pointed projection on the other side and located between the first two, the pointed projections being dimensioned so as to enable the locking piece to be forced into the open side of the cavity after the cover, and to secure the cover therein.

References Cited UNITED STATES PATENTS 1,669,447 5/1928 Boyton et al. 17466 3,107,336 10/1963 Caddock et al. 338 3,175,723 3/1965 Schmidt et al. 17450 X 3,187,289 6/1965 Rolwes 338-180 ROBERT K. SCHAEFER, Primary Examiner.

J. HOHAUSEN, Assistant Examiner, 

